The present invention relates generally to electrostatic actuators and more particularly to electrostatic mnicroactuators with comb drive assemblies.
Electrostatic comb drive microactuators have heretofore been provided. See, for example, U.S. Pat. Nos. 5,025,346 and 5,998,906. Flexural suspensions for such microactuators have generally fallen into there categories: fixed-fixed beams, crab-leg flexures and folded flexures. For a discussion of these suspensions, see G. Legtenberg, et al., xe2x80x9cComb-Drive Actuators for Large Displacementsxe2x80x9d, J. Micromech. Microeng. 6 (1996), pp 320-329. Folded flexures are further described in U.S. Pat. No. 5,025,346 and Michael Judy""s U.C. Berkeley dissertation, xe2x80x9cMechanisms Using Sidewall Beamsxe2x80x9d, 1994.
The maximum motion of electrostatic comb drive microactuators is often limited by electromechanical side instability forces which occur during interdigitation. In this regard, undesirable sidewise movement and possible snapover of the comb drive fingers can result from such side instability forces. Flexural suspensions can be utilized to discourage such sidewise movement. A discussion of this behavior, particularly with respect to fixed-fixed beams, crab- leg flexures and folded flexures, is set forth in the G. Legtenberg, et al. article cited above.
Several notable solutions for minimizing sidewise movement or snapover of comb drive fingers in linear electrostatic microactuators are set forth in U.S. Pat. No. 5,998,906. The linear comb drive assemblies described in the ""906 Patent are disposed between first and second folded-beam suspensions, which enhance alignment of the comb drive fingers during deflection and thus minimize nonlinear travel of the comb drive fingers during deflection. Each of the folded-beam suspensions therein consists of a pair of beams connected in series. The pair of beams of each folded-beam suspension are connected at one end to a common bar. The opposite ends of such beams are connected either to a movable shuttle or to the fixed substrate. The compliance of a folded-beam suspension in the sideways direction results from two effects caused by the side load, namely individual beam extension or contraction in the sideways direction and beam distortion in the forward direction. The first term is mechanical and the second term is geometric. The stiffness of the suspension is the inverse of the compliance and thus the combination of the mechanical and geometric terms.
A nonfolded flexure suspension for an electrostatic actuator using parallel plate electrodes is described in R. Brennen, xe2x80x9cLarge Displacement Linear Actuatorxe2x80x9d, 1990 technical digest of IEEE conference on Micro Electro-Mechanical Systems, pp 135-139. The suspension beams, connected by a shuttle, are initially inclined relative to the parallel plate electrodes by an angle of approximately five degrees. The load to the suspensions is applied normal to the shuttle. The motive force is produced by the increase in the projected length of the suspension beams, which reduces the electrostatic gap between the plates.
In general, it is an object of the present invention to provide an electrostatic microactuator having an improved suspension.
Another object of the invention is to provide an electrostatic microactuator of the above character having improved side stability.
Another object of the invention is to provide an electrostatic microactuator of the above character having a suspension that provides side stiffness to the comb drive fingers that is substantially independent of the forward deflection of the microactuator.
Another object of the invention is to provide an electrostatic microactuator of the above character having nonfolded suspension members.
Another object of the invention is to provide an electrostatic microactuator of the above character having reduced size and complexity.
Another object of the invention is to provide an electrostatic microactuator of the above character having a comb drive assembly with comb teeth that are inclined relative to the comb drive bar.
Another object of the invention is to provide an electrostatic microactuator of the above character having a comb drive assembly with movable comb teeth that are offset from the midpoint between the stationary comb teeth of the comb drive assembly.
The present invention provides an electrostatic microactuator comprising a substrate and at least one comb drive assembly having first and second comb drive members. The first comb drive member is mounted on the substrate and the second comb drive member overlies the substrate. At least one spring member is provided and has a first end portion secured to the substrate and a second end portion secured to the second comb drive member. The first comb drive member has a first elongate member and a plurality of spaced-apart first comb drive fingers extending from a side of the first elongate member with respective spaces therebetween. The second comb drive member has a second elongate member and a plurality of spaced-apart second comb drive fingers extending from a side of the second elongate member. The first comb drive member has a midpoint in the space between each adjacent pair of the first comb drive fingers. The second comb drive member is movable between a first position in which each second comb drive finger is not substantially fully interdigitated with an adjacent pair of first comb drive fingers and a second position in which each such second comb drive finger is substantially fully interdigitated with such adjacent pair of first comb drive fingers. Each of the second comb drive fingers is offset relative to the second elongate member from the midpoint between the adjacent pair of first comb drive fingers when in the first position and is substantially centered on such midpoint when in the second position.